1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and an electrophotographic apparatus using such a member and, more particularly, to an electrophotographic photosensitive member and an electrophotographic apparatus which are not susceptible, or not readily susceptible, to unevenness in image density even when there arises uneven abrasion (non-uniform wearing).
2. Related Background Art
In an electrophotographic apparatus, such as a copying machine, a facsimile or a printer, the peripheral surface of a photosensitive member, on which a photoconductive layer is formed, is uniformly charged by charging means such as corona charging, roller charging, fur brush charging or magnetic brush charging; then an electrostatic latent image is formed on the peripheral surface of the photosensitive member by exposure of a copied image of an copying object with laser or LED light according to a reflected light or modulated signal; a toner image is formed by adhering a toner to the photosensitive member; and the toner image is transferred to a sheet of copying paper or the like to form a copied image.
After a copied image is formed in the electrophotographic apparatus in this manner, there remains on the peripheral surface of the photosensitive member a part of the toner, and the residual toner needs to be removed. Usually, the residual toner is removed by a cleaning step using a cleaning blade, a fur brush, a magnetic brush or the like.
In recent years, from the viewpoint of consideration for environment, there have been proposed and introduced to the market electrophotographic apparatuses in which the cleaning device is dispensed with to reduce or eliminate a waste toner. They include what is disclosed in the Japanese Patent Application Laid-Open No. 6-118741, in which a direct charger, such as a brush charger, also serves the cleaning purpose, and what is disclosed in the U.S. Pat. No. 6,128,456, in which a developer also takes charge of cleaning, but both involve a step in which the toner and the surface of the photosensitive member are worn to remove the toner.
However, the need for high level of picture quality of printed images in recent years has led to the use of a toner smaller in average grain size than what was previously used or a toner with a lower melting point that is compatible with energy conservation, and there has been occurred the phenomenon that such a toner will be fusion bonded to a surface of a photosensitive member. In a toner removing process for removing the toner at an initial stage of fusion bonding, there is a case where the increase of load imposed on the cleaning step generates uneven abrasion of a surface layer of a photosensitive member or where an unevenly located charging member remains in contact with a surface layer of a photosensitive member to generate uneven abrasion of the surface layer.
Thus, there has been a problem that irradiation with an image exposure light in such a condition will generate interference due to unevenness in the thickness of a surface layer, which in turn will give rise to a difference in quantity of light incident on a photoconductive layer to generate belt-like unevenness in a halftone image. Moreover, along with the increasing digitization of electrophotographic apparatuses in recent years, latent image formation with a light source mainly emitting a light of a single wavelength is becoming the main stream, which results in frequent occurrence of interference, thereby aggravating the problem.
With a view to solving this problem, as disclosed in the Japanese Patent Publication No. 5-49108 and U.S. Pat. No. 4,795,691, there are proposed methods to prevent the halftone image unevenness caused by unevenness in the quantity of incident light attributable to uneven abrasion of a surface layer by providing an intermediate layer between a photoconductive layer and a surface layer of a photosensitive member with a photosensitive layer of amorphous Si or by continuously varying the composition of the interface to thereby reduce or eliminate reflection at the interface.
Whereas recently introduced digital copying machines and printers use such a photosensitive member, they are often inadequate for preventing unevenness in halftone images arising from unevenness in film thickness of fine pitches, ranging from tens of xcexcm to a few mm attributable to the aforementioned cleaner or contact charger. On the other hand, the configuration to continuously vary the interface composition to effect control so as to restrain interface reflection at that part, requires strict control of the manufacturing conditions to achieve steady production by reducing fluctuations in characteristics within and between individual photosensitive members and, moreover, involves such a delicate aspect that, where the composition of a photosensitive member has changed, the optimal continuous interface is determined by a balance of various characteristics.
Further, Japanese Patent Application Laid-Open No. 11-2996 proposes to polish a photosensitive member to regulate the surface roughness Rz to a predetermined value. However, no attention is paid to the occurrence or prevention of halftone image unevenness arising from unevenness in film thickness of fine pitches, ranging from tens of xcexcm to a few mm attributable to a cleaner or contact charger.
Along with the increasing digitization of electrophotographic apparatuses in recent years, latent image formation with a light source mainly emitting a light of a single wavelength, such as a laser or an LED array, is becoming the main stream, but at the same time the speed of copying, i.e. the number of revolution of the photosensitive member, keeps on increasing along with the advancement of electric circuit elements. As a result, by merely relying on the method of reducing or eliminating reflection at the interface by provision of an intermediate layer between the photoconductive layer and the surface layer of a photosensitive member or continuously varying the composition of the interface, there arises a difference in the quantity of exposure light incident on the photoconductive layer, due to interference by the single wavelength light due to uneven abrasion of the surface layer, thereby sometimes generating a belt-like density difference in the printed image.
Further, the new addition of a step of previously roughing the surface of the conductive substrate will increase the production cost. Machining the substrate with such a roughness as to generate no density difference may pose a new problem of lowering in the image sharpness.
The present inventors have conducted extensive studies and found that the effect of preventing the belt-like (or linear) unevenness in a halftone image due to uneven abrasion of the surface layer is not determined merely by the control of the interface composition or the substrate roughness, but also greatly depends on the microscopic surface roughness (more specifically in the order of a few nm to tens of nm) peculiar to the surface of the a-Si (amorphous silicon) photosensitive member.
An object of the present invention, completed on the basis of the above described findings, is to provide a photosensitive member and an image forming apparatus that successfully ensure formation of a satisfactory image by preventing fusion bonding of a toner during cleaning.
According to the present invention, there is provided an electrophotographic photosensitive member formed by successively stacking on a conductive substrate a photoconductive layer comprising amorphous Si and a surface protective layer comprised of an amorphous material, wherein the minimum value (hereinafter referred to as Min) and the maximum value (hereinafter referred to as Max) of the reflectance (%) of the photosensitive member within the wavelength range of 600 nm to 700 nm satisfy the relation of 0xe2x89xa6(Maxxe2x88x92Min)/(Max+Min)xe2x89xa60.20, and a center line average roughness Ra1 of the interface on the surface side of the photoconductive layer and a center line average roughness Ra2 of the outermost surface of the surface layer, within the range of 10 xcexcmxc3x9710 xcexcm, satisfy the relations of Ra1/Ra2xe2x89xa71.3 and 22 nmxe2x89xa6Ra1xe2x89xa6100 nm, and an electrophotographic apparatus having the electrophotographic photosensitive member.
The inventors have found that this makes possible to prevent a toner from fusion bonding to the surface of a photosensitive member to ensure formation of a satisfactory image, and succeeded in completing the present invention.
The term xe2x80x9cmicroscopic surface roughnessxe2x80x9d as used herein refers to the value of surface roughness Ra measured by using an atomic force microscope (AFM) (trade name: Q-SCOPE 250 mfd. by Quesant). In order to measure microscopic surface roughness with high accuracy and good reproducibility, it is desirable to measure the roughness within the measuring range of 10 xcexcmxc3x9710 xcexcm in such a manner as to avoid any error due to the curvature tilt of the sample. To be more specific, this can be accomplished by parabolic correction whereby the curvature of the AFM image of the sample is fitted to a parabola in the tile removal mode of Quesant""s Q-SCOPE 250 and then flattening is effected. This is an appropriate method because an electrophotographic photosensitive member usually has a cylindrical shape.
Further, if the image remains inclined, another procedure of correction (line by line) to remove the inclination is carried out. Thus, it is possible to appropriately correct any inclination of the sample within such a range as to generate no distortion of the data.
The term xe2x80x9ccenter line average roughness Ra within a range of 10 xcexcmxc3x9710 xcexcmxe2x80x9d as used herein refers to a value calculated from a three-dimensional shape by Quesant""s atomic force microscope (AFM) Q-SCOPE 250 (Version 3.181).
When the present inventors calculated the two-dimensional center line average roughness Ra of a random sectional curve from a three-dimensional shape measured with the atomic force microscope, it was in substantial agreement with the centerline average roughness Ra within the range of 10 xcexcmxc3x9710 xcexcm calculated from the three-dimensional shape. However, the Ra value obtained from the three-dimensional shape is more desirable in terms of the stability of measurements and the mechanism of interference generation.
In the present invention, the means to establish the fine roughness relation Ra1/Ra2xe2x89xa71.3 for disturbing the degree of parallelization of the surface layer includes not only the later described control of the film forming conditions for a photosensitive member or selection of the surface material but also, if necessary, further polishing to a desired level of fine roughness by the photosensitive member surface treating method such as described in Japanese Patent Publication No. 7-77702. More specifically, the conceivable method includes bringing a lapping tape available from Fuji Photo Film Co., Ltd., or 3M Co. into contact under pressures to a rotating photosensitive member to polish the surface thereof.
In particular, Ra1 is controlled by the degree of roughing by surface treatment of the substrate and the preparation conditions of the photoconductive layer, specifically, the ratio of source gases, gas flow rates, substrate temperature and discharge power. Ra2 is controlled by the preparation conditions of the surface layer, specifically, the ratio of source gases, gas flow rates, substrate temperature, discharge power and steps accompanied with surface polishing as an after-treatment or polishing in an electro-photographic apparatus.
 less than Fine Surface Roughness of Surface Side Interface of Photoconductive Layer and Outermost Surface of Surface Layer, and Degree of Parallelization of Surface Layer greater than 
The fine degree of parallelization of the surface layer portion in the present invention will be described below.
An atomic force microscopy has a horizontal resolving power (resolving power in a direction parallel to the sample surface) finer than 0.5 nm and a vertical resolving power (resolving power in a direction perpendicular to the sample surface) of 0.01 to 0.02 nm, and is capable of measuring the three-dimensional shape of a sample. It is significantly distinguished from any surface roughness gauge, which is already in extensive use, in its high resolving powers.
Incidentally, in performing measurement with an AFM, the present inventors have measured a number of samples with a number of scanning sizes. The term xe2x80x9cscanning sizexe2x80x9d is the length of a side of a square that is scanned. Therefore, a scanning size of 10 xcexcm means scanning of a range of 10 xcexcmxc3x9710 xcexcm, i.e. 100 xcexcm2. A part of the measurement result is shown in FIG. 1, in which the horizontal axis of the graph represents the scanning size. FIG. 1 shows an example of the range of data obtained with a single scanning size.
When the scanning size is enlarged, i.e. the range of measurement is expanded, the measurements will become more stable, but the affection of the specific shapes such as waviness or projection of a sample substrate, or the machined shape will make it more difficult for the fine shape to be reflected, while a narrower angle of visibility increases fluctuations by selection of parts to be measured, so that the present invention has adopted the representation in terms of a 10 xcexcmxc3x9710 xcexcm field of view, which is synthetically excellent in the detection capacity of measurement and the stability. It should be understood from the above circumstances that the idea underlying the present invention is not limited to a 10 xcexcmxc3x9710 xcexcm field of view.
With so high resolving powers, it is possible to measure not just the roughness in an order where the roughness of the photosensitive member substrate is the dominant factor, but even such types of roughness attributable to the nature of deposited films themselves, such as a photoconductive layer, a surface layer, etc.
While the roughness of a photosensitive member substrate is dependent on xe2x80x9cpatternsxe2x80x9d, including the xe2x80x9ctreated memberxe2x80x9d and xe2x80x9ctooth profilexe2x80x9d such as what results from lathing, ball milling or dimpling, the roughness of a deposited film themselves has no pattern but involves complex profile factors.
One example of observed images is shown in FIG. 2. Details will be given afterwards with reference to Experiments and Examples of the invention.
Regarding the interference of the surface layer, the inventors have suspected that not only the parameter of the surface layer thickness in submicron order but also the parallelization of the surface layer, in which the very fine surface roughness of the surface side interface of the photoconductive layer and the outermost surface of the surface layer are reflected, may play a major part, and verified their suspicion through analysis.
More specifically, using a field emission type scanning electron microscope (FE-SEM) (Model S-4200 mfd. by Hitachi, Ltd.), samples were observed, which were subjected sectioning treatment with a focused ion beam (FIB) (FIB-200 type FIB apparatus mfd. by Fei Co.).
Examples of observed images are shown in FIGS. 3A through 3D and 4A through 4D.
The sample shown in FIG. 3A is an observed sectional image (xc3x9710000) of the surface layer portion in accordance with the present invention; FIG. 3B is an enlarged image (xc3x9750000) of a part near the boundary of the layers; FIGS. 3C and 3D are views more clearly illustrating the outline of the layers observed in FIGS. 3A and 3B, respectively. As is seen from FIGS. 3A through 3D, the roughness of the outermost surface of the surface layer, corresponding to the Ra2 value according to the invention, is smaller than the roughness of the surface side interface of the photoconductive layer corresponding to the Ra1 value according to the invention. In contrast thereto, in the samples shown in FIGS. 4A through 4D (drawn by following the same procedure as FIGS. 3A through 3D), the roughness of the outermost surface of the surface layer is approximately equal to that of the surface side interface of the photoconductive layer, i.e. substantially in parallel to the fine surface shape. Detailed comparison of numerical values will be made afterwards with reference to Experiments and Examples of the invention.
 less than Relationship between Surface Layer Thickness and Sensitivity greater than 
It is preferable that the surface spectral reflectance of the aforementioned photosensitive member satisfies the conditions represented by the following equations.
For Min and Max of the reflectance (%) within the wavelength range of 600 nm to 700 nm:
0xe2x89xa6(Maxxe2x88x92Min)/(Max+Min)xe2x89xa60.20
more preferably,
0xe2x89xa6(Maxxe2x88x92Min)/(Max+Min)xe2x89xa60.10
still more preferably,
0xe2x89xa6(Maxxe2x88x92Min)/(Max+Min)xe2x89xa60.05
Herein, the term xe2x80x9creflectancexe2x80x9d as used herein refers to a reflectance (percentage) measured with a spectrophotometer (trade name: MCPD-2000 mfd. by Otsuka Denshi Co.). To outline the measuring process, first the spectral emission intensity I(O) of the light source of the spectrophotometer is measured, then the spectral reflectance intensity I(D) of the photosensitive member is measured, and the reflectance R=I(D)/I(O) is calculated. For accurate measurement with good reproducibility, it is desirable to fix the detector with a jig so as to keep a constant angle relative to the photosensitive member having a certain curvature.
Specific examples of control of degree of parallelization are shown in FIGS. 5A and 5B. FIG. 5A shows a wavelength range of 400 to 720 nm, and FIG. 5B, a wavelength range of 600 to 700 nm. The data are the same for both diagrams. Data A and B are examples in which the degree of parallelization (or the property to be equidistant from each other) between the (photoconductive layer)/(surface layer) interface and the outermost surface is good, while data C, D and E are examples in which the degree of parallelization between the (photoconductive layer)/(surface layer) interface and the outermost surface is disturbed.
It is to be further noted that data A, B and C are examples outside the scope of the present invention.
The presence of two lines of data A and B is due to a difference in the film thickness of the surface protective layer, and the waveforms shift laterally on the graph depending on the difference in film thickness. As their maximum values correspond to the amplitudes of waveforms, those which show good degree of parallelization between the (photoconductive layer)/(surface layer) interface and the outermost surface, as viewed when fixed in a single wavelength, vary more greatly in reflectance than those which show disturbed degree of parallelization, with variation of the film thickness. That is, there arise a great variation in sensitivity along with the variation in the film thickness.
On the other hand, for data C, D and E, since Ra2 is changed to disturb the degree of parallelization between the (photoconductive layer)/(surface layer) interface and the outermost surface, the variation is significantly small.
Furthermore, in data D and E, which are examples of the present invention, the variations are almost negligible, and even when uneven abrasion of the surface layer of the photosensitive member arises in the cleaning step or an unevenly located charging member remains in contact with the surface layer of the photosensitive member to generate uneven abrasion of the surface layer, it is possible to prevent occurrence of an image unevenness.
 less than Relationship between Uneven Abrasion of Surface Layer and Unevenness in Halftone Image Density greater than 
On the basis of the aforementioned result of the analysis and the electrophotographic evaluation, the mechanism of occurrence of unevenness in halftone image density and that of the effect of the present invention will now be described with reference to FIGS. 6A and 6B.
As described so far, Ra1 and Ra2 are substantially equal on the surface of an a-Si photosensitive member because of its production method, with the result that the surface layer thickness is constant from part to part, i.e. the surface is substantially parallel to the interface between the surface layer and the photoconductive layer. Since a light incident on the surface is reflected by the interface between the surface layer and the photoconductive layer and interferes with a light reflected from the surface, the quantity of incident light will be determined by the thickness of the surface layer according to the principles of interference. That is, a difference in the film thickness provides a difference in the electric potential, which is reflected in the image. This was as explained with reference to FIGS. 5A and 5B.
In practice, a portion of uneven abrasion will be generated in the surface layer as illustrated in FIG. 6A, and in whatever form the uneven abrasion may arise, the conditions for interference are met at least in a portion other than the uneven abrasion portion, so that the difference in the quantity of incident light at that portion differ from that at the uneven abrasion portion, thus giving rise to image unevenness.
However, in a photosensitive member as shown in FIG. 6B wherein the relationship between the photoconductive layer and the surface layer is Ra1/Ra2xe2x89xa71.3, more preferably Ra1/Ra2xe2x89xa71.5, and still more preferably Ra1/Ra2xe2x89xa72.0, the conditions for interference are not met, and the electric potential does not depend on the thickness of the surface layer. Incidentally, by setting Ra1 to 22 nm or more, more preferably 30 nm or more, occurrence of interference can be prevented, and occurrence at such a portion of any flaw or linear abrasion that might be reflected in the image can also be prevented.
Controlling Ra2 by appropriately setting the conditions of surface layer formation or by proper after-treatment to achieve a relationship of Ra1/Ra2 less than 1 also has an effect to disturb the degree of parallelization, but the conditions for interference may come to be met during use because of decrease of Ra2 by endurance printing, it is preferable to manufacture the product within the range where the conditions for interference can never be met from the outset, i.e. Ra1/Ra2xe2x89xa71.3, more preferably Ra1/Ra2xe2x89xa71.5, or still more preferably Ra1/Ra2xe2x89xa71.8.
When Ra1 is to be controlled by machining the substrate, the substrate face and the surface also become approximately parallel to each other, the interference between them is not negligible. Since the photoconductive layer is highly absorbent unlike the surface layer, in order not to allow a light reflected by the substrate from interfering with a light reflected by the surface, it is preferable to select the photoconductive layer thickness or the light wavelength so as to provide sufficient light absorption so that the lights reflected from the substrate may not return to the surface.
Although depending on the exposure light wavelength and the absorption coefficient of the photoconductive layer, within the exposure light wavelength range which now constitutes the main stream, interference between the substrate and the Ra1 face can be prevented by setting the film thickness to 14 xcexcm or more, more preferably 20 xcexcm.
On the other hand, by setting the film thickness to 50 xcexcm or less, Ra1 is made more controllable, and the peeling off of the film, increase of image defects and increase of production cost, that might arise where control is difficult, can be prevented from occurring.
Therefore, the film thickness of the photoconductive layer of the aforementioned photosensitive member is preferably 14 to 50 xcexcm, more preferably 20 to 50 xcexcm.
For the microscopic surface roughness in the present invention, the aforementioned Ra value of surface roughness measured using an atomic force microscope (AFM) (trade name: Q-SCOPE 250 mfd. by Quesant) is easier to handle, and, in order to measure the microscopic surface roughness with high accuracy and good reproducibility, it is desirable to measure the roughness within the range of 10 xcexcmxc3x9710 xcexcm. Further, in order to measure Ra1 of a photosensitive member having layers including the surface layer formed therein, there also is available an alternative method by which a calibration curve is prepared from the relationship between surface roughness obtained by observing a section of the photosensitive member with FE-SEM, TEM or the like and surface roughness obtained with AFM, and Ra2 is substituted with the roughness up to the photoconductive layer obtained by sectional observation.